The invention relates to a valve arrangement in a hydraulic circuit, its use and also an arrangement for controlling a hydraulic vehicle drive.
In hydraulic control systems there is a need for a valve arrangement which acts as a throttling valve when the pressure of the hydraulic fluid flowing through exceeds a specific threshold value. One important application of this is arrangements for controlling hydraulic vehicle drives. The latter generally have a closed hydraulic circuit which has a variable displacement pump driven by a diesel engine, a hydraulic motor driving the drive wheels of the vehicle and two lines connecting the variable displacement pump and the hydraulic motor for the hydraulic fluid, it being possible for each of the lines, depending on the direction of travel of the vehicle, to be the flow or the return line for the hydraulic motor (DE 199 30 997 A1). In hydraulic circuits of this type, the hydraulic motor can act as a pump when the vehicle changes from the normal driven state to trailing or overrun operation. The vehicle then attempts to accelerate the variable displacement pump and therefore also the diesel engine, which can lead to damage to the diesel engine.
For this reason, it has already been proposed, according to DE 199 30 997 A1, to insert into the return line of the hydraulic circuit a pressure regulating valve which acts as a throttle and which acts in the event of increased pressure of the hydraulic fluid flowing from the hydraulic motor to the variable displacement pump. In the pressure regulating valve acting as a throttle, the energy of the hydraulic fluid flowing from the hydraulic motor to the variable displacement pump is partly converted into heat, which means that further undesired overrunning of the diesel engine is prevented. If, during normal driving operation, the usual lower pressures are present in the return line, the throttling action remains nonfunctional. However, if the direction of travel of the vehicle is reversed, the return line then has to serve as a flow line, which is under increased pressure and in which a throttling action would be undesirable. For this reason, in the known vehicle drive according to DE 199 30 997 A1, a line with a nonreturn valve is provided in parallel with the valve arrangement, so that the pressure regulating valve remains inactive when the flow direction is reversed. If it is intended to secure the vehicle against overrunning of the diesel engine in both directions of travel, then the arrangement described of the pressure regulating valve with the parallel-connected nonreturn valve is required in a dual arrangement. This parallel connection of the two functions by means of valves which are designed separately and independently of each other needs a great deal of installation space or leads to a restriction of the flow cross sections, associated with corresponding hydraulic-mechanical power losses.
The invention is therefore based on the object of providing a valve arrangement which acts as a throttle in only one flow direction, with which installation space is saved and at the same time the effort for mounting is reduced, in addition the greatest possible flow cross sections becoming possible in a small available installation space.
According to the invention, this object is achieved in by a valve arrangement in a hydraulic circuit, having an L-shaped duct arrangement, in which a throttling element whose length can be adjusted can be displaced in the direction of one arm of the L-shaped duct arrangement in accordance with the position of a pilot control valve, the adjustment of the length of the throttling element being made under the influence of the pressure of the hydraulic fluid entering its interior and the design being such that the throttling element is pushed into the duct arrangement when a specific threshold pressure of the hydraulic fluid is exceeded, irrespective of the flow direction of the hydraulic fluid, but its length is shortened when the inflow to its end takes place at a pressure higher than the threshold pressure.
The valve arrangement according to the invention is distinguished by the fact that hydraulic fluid under increased pressure can flow through it in both flow directions but a throttling action occurs only in one flow direction. The throttling action does not occur if the inflow against the adjustable length throttling element is from one end. In this case, the throttling element automatically adjusts to a shorter length. A throttling action is prevented if, in accordance with the increased pressure of the hydraulic fluid, the throttling element assumes its extended position overall. In this way, a line led in parallel and having a nonreturn valve can be saved.
With the valve arrangement according to the invention, the same action is thus achieved as with the nonreturn valve located in a bypass line in an arrangement according to the prior art. However, the outlay on construction is lower; space is saved and larger flow cross sections can be implemented.
The fact that a pilot control valve is provided in the valve arrangement according to the invention presents no obstacle to this, since the said pilot control valve is expedient in any case for registering a specific threshold pressure of the hydraulic fluid.
Advantageous refinements and structural details of the valve arrangement according to the invention are specified in the claims that refer back.
The invention also covers the use of a valve arrangement in a closed circuit of a hydraulic drive.
In this case, this can be, for example, the drive of a machine tool which is equipped with a hydraulic transmission. A particularly preferred field of application of the valve arrangement according to the invention is, however, vehicle drives. Therefore, the invention also extends to the arrangement for controlling a hydraulic vehicle drive.
The arrangement for controlling a hydraulic vehicle drive, according to the invention, comprises a closed hydraulic circuit which has a variable displacement pump driven by a diesel engine, a hydraulic motor driving the drive wheels of the vehicle and lines connecting the variable displacement pump and the hydraulic motor for the hydraulic fluid, it being possible for each of the lines to be the flow or return line of the hydraulic motor, depending on the direction of travel of the vehicle, a valve arrangement being provided in at least one of the lines.
If the vehicle is to be secured against impermissibly high rotational speeds of the diesel engine in overrun operation in both directions of travel, a valve arrangement according to the invention must be incorporated in each of the lines.